America’s homicide clearance rate—the percentage of solved
crimes that lead to arrest—has fallen considerably in the
past 50 years, from around 90% in 1965 to around 64% in
2012, according to federal statistics.
(https://www.economist.com/news/united-states/21656725-police...)
(See, also, https://www.citylab.com/equity/2017/06/police-arent-getting-...)IMO, similar to counter-terrorism efforts (most spectacularly, 9/11), technology becomes a crutch. Frankly, I wouldn't be surprised if mass encryption leads to _improved_ clearance rates, as law enforcement becomes less complacent. Take this latest example: the FBI agent declines Apple's help because he's convinced the geeks at the FBI lab can handle it. He presumably doesn't bother actually confirming with the geeks in the lab, nor does he attempt to put Apple in touch with the lab. Just utter complacency, confident that the machine (computers, bureaucracy) will suffice.
This sort of laziness wouldn't have been tolerated in Hoover's FBI. Moreover, Hoover likely would have been more aggressive using the tools available to him--keyloggers, etc--rather than whining.
Regarding Japan, yes, their official rates are questionable. I wish I could find the journal article, but last year I was researching the supposed 100% clearance rate in Singapore and came across a very in-depth article[1] that discussed clearance rates in Singapore, Japan, and elsewhere in Asia and that left me with the impression that w'ever the actual clearance rates, they're nonetheless _much_ better than the U.S.
[1] Spoiler: the 100% clearance rate in Singapore was plausible, in no small part because it's a small city-state with very few homicides to begin with.